A touch sensor may detect the presence and location of a touch or the proximity of an object (such as a user's finger or a stylus) within a touch-sensitive area of the touch sensor overlaid on a display screen, for example. In a touch-sensitive-display application, the touch sensor may enable a user to interact directly with what is displayed on the screen, rather than indirectly with a mouse or touch pad. A touch sensor may be attached to or provided as part of a desktop computer, laptop computer, tablet computer, personal digital assistant (PDA), smartphone, satellite navigation device, portable media player, portable game console, kiosk computer, point-of-sale device, or other suitable device. A control panel on a household or other appliance may include a touch sensor.
There are a number of different types of touch sensors, such as (for example) resistive touch screens, surface acoustic wave touch screens, and capacitive touch screens. Herein, reference to a touch sensor may encompass a touch screen, and vice versa, where appropriate. When an object touches or comes within proximity of the surface of the capacitive touch screen, a change in capacitance may occur within the touch screen at the location of the touch or proximity. A touch-sensor controller may process the change in capacitance to determine its position on the touch screen.
A display screen includes a number of layers that form a display stack. The layers of the display stack enable the display screen to produce a color image. The number and type of layers depends on the type of display screen. For example, a Liquid Crystal Display (LCD) based display screen has different layers than an Organic Light Emitting Diode (OLED) based display screen. To form a touch screen, a touch sensor is typically placed over the display stack. For example, the touch sensor may be formed on a transparent cover. The transparent cover, with the touch sensor, is then placed over an already formed display stack. This arrangement negatively impacts the contrast ratio of the display screen. For example, there is typically an air gap between the display stack and the sensor which can create undesirable reflections.